1. Field of the Invention
The present invention relates to blades for rotary lawn mowers that utilize a blade that rotates in the horizontal plane and particularly to blades that utilize radially-extending cutting elements in lieu of the standard bar blade.
2. Prior Art
The typical lawn mower blade consists of a group of sharpened metal plates that rotate about an axis in the horizontal plane. The principal disadvantages of these older devices, namely, that they are dangerous to the user and hurl objects great distances at high velocity, have been addressed in lawn mower blades that employ filaments or similar elements that extend radially from the axis of rotation instead of the bar blade sharpened on the ends. The claimed advantages of such devices include the inability of a flexible cutting element to hurl a heavy object and the fact that filaments will do less harm to an operator's foot than a metal blade. In addition, filaments that structurally fail while in use do not constitute the same dangerous high-speed objects that a metal blade can become. For example, if a lawn mower is pushed over a rock or metal sprinkler head, the blade itself may break into several pieces which can be hurled at speeds of 200 miles per hour.
While filament or flexible cutting element type blades are improvements over the older "fan-like" metal blades, one principal disadvantage of these devices is the short lifetime of filaments, usually made of a plastic such as nylon.
In the event that the lawn mower is used over an area containing various objects such as pine cones or rocks, devices in the prior art that use flexible cutting elements rely on the fact that the filament will simply bend in the horizontal plane on impact and then bend back to the straight cutting position when the obstacle has passed. Alternatively, the cutting element filaments may be frictionally engaged via a mounting post on the rotor body that is used. This arrangement allows the filament to move in the horizontal plane upon impact with an object such as a rock but do not provide a simple method for the cutting element to return to its cutting position. This problem arises because the radial forces on the filament cannot cause the element to rotate against the frictional forces. That is to say, the filaments are not pivotably mounted on the rotating rotor blade. This problem exists with the blade discussed in U.S. Pat. No. 3,208,209. In U.S. Pat. No. 3,208,209 wires are used as cutting elements and do not rotate around their respective mounting posts during a normal cutting operation. If the cutting elements are rotated because of impact with foreign objects, it would be necessary to stop the mower and return the elements to their normally radially extending position by loosening the mounting bolts, manually returning the cutting elements to their normal position and then re-tightening the bolts. Such a procedure is time-consuming at the very least. Furthermore, if the respective mounting bolt is not adequately tightened there exists the distinct possibility that the filaments could come loose and be thrown by the rotating body or knocked off the respective mounts in subsequent collisions. Also, the replacement of the cutting elements is a time-consuming job involving the unloosening of a plurality of bolts and replacement of the corresponding element. Finally, the cost and weight of a lawn mower blade are important considerations and the use of a bolt for each respective cutting element should not be required.
What is most desirable in a lawn mower blade is a device where the cutting elements are freely pivotable in the horizontal plane and also pivotable in the vertical plane. Thus, a cutting element can ride up and over a particular foreign object or pivot backwards in the event of a collision and quickly return to the normal horizontally extending cutting position after the object has passed. The improved lawn mower blade should be at least equal to or lighter in weight than conventional blades and should cost no more. In addition, and very importantly, the cutting elements should be practically impossible to install incorrectly and should be installable quickly and easily. The blade assembly should have no protrusions on the edges or lower surface that can engage objects or injure an operator's foot that is accidentally placed underneath the mower housing. Such protrusions may be connecting bolts, nuts or the like which extend below a solid rotor or plate. Also, the cutting elements should be quickly and easily installable without the aid of tools and without having access to the upper surface of the rotor. None of the devices known or disclosed in the prior art are satisfactory from the view of cost, weight, safety and ease of use.